It is often necessary to control the resistance of one component of an electrical device with respect to another. An example is found in computer disc drives. In this application it is often critical to maintain a specific resistance between the read-write head of the disc drive and the magnetic media disc thereof. This is necessary to prevent any electrical flow between the head and the disc. If the resistance between these two components is too high, an electrostatic charge can build up between the two, which charge can result in an electrostatic discharge from head to disc or vice versa. This discharge will generally result in a loss of data stored on the disc, and will likely result in such damage to the read--write head and/or the disc as to require replacement of the disc drive.
On the other hand, if the resistance between the head and the disc is too low, a similar but different problem occurs. The separation between the head and disc, during operation, is typically measured in micro-inches (.mu.in). If there exists a difference in electrical potential between the head and disc, insufficient resistance therebetween can cause an electrical current to flow across the gap between the two components.
In at least one disc drive application, it is a goal of the drive's designers to maintain a resistance of approximately 1 meg.OMEGA. (at less than 1 volt potential) between the head and disc of a disc drive. By generally maintaining this level of resistance between the disc and the head, the twin problems of electrostatic discharge and current flow between the two components are generally eliminated.
The problem in maintaining the previously discussed specific resistance lies in maintaining a reliable current path between the rotor of the spindle motor, which drives the disc, and the motor stator which, in many designs, is electrically connected with the arm which supports the read--write head. In many electrical applications, this connectivity between rotating and non-rotating components is accomplished through the use of slip rings, spring-loaded contact fingers and the like.
U.S. Pat. No. 4,701,653 describes a spring finger grounding methodology applied to the end of the shaft of an electric motor for use in a disc drive. U.S. Pat. No. 5,251,081 describes a spring finger grounding methodology applied to the body of the shaft of an electric motor, also for use in a disc drive. U.S. Pat. No. 3,691,542 describes a slip ring grounding methodology applicable to disc drive technology.
In the case where an electric spindle motor is sealed within a disc drive, effective connectivity between the motor's rotor and its stator is both increasingly important and increasingly difficult. Slip rings, contact fingers and the like are essentially friction devices. This friction has several adverse effects in disc drive spindle motor applications.
A first adverse effect of the friction between the contacting elements is that friction can result in abrasion. This abrasion generally leads to particulate matter being abraded from at least one of the contacting elements. Given the extremely small tolerances inherent in current art disc drive technology, these abraded particles can quickly lead to a catastrophic failure of the disc drive.
Motors for use in current disc drive designs are usually quite small: the entire motor is often no larger than 3 mm.times.25 min. Due to these extreme size limitations, contact fingers and slip rings must be mounted very close to the motor bearings. Particulate matter introduced into these very small precision bearings causes increased acoustic noise and significant bearing wear leading to failure of the drive.
The particulate matter may also migrate onto the surface of the disc or the read--write head. Given the relatively large size of the abraded particles relative to the clearance between the head and the disc, introduction of any particulate matter onto either of these components will almost inevitably result in the instant failure of the drive.
Effective connectivity between the disc drive spindle motor rotor to the stator thereof is made more difficult because, in order to operate properly, disc drives are typically sealed in order to prevent the incursion of contaminants into the disc drive's internal workings. This sealing of the internal components of a disc drive makes repair of these devices essentially impossible. Because it is a principal requirement and aim of the designers and manufacturers of disc drive equipment to increase the mean time between failures (MTBF) of their product, the internal components of a disc drive are required to function for many thousands of hours without any repair or maintenance whatever.
A second adverse effect of the friction devices previously discussed is that the electrical connectivity they provide is not stable over time. Corrosion and abrasion products which may accumulate between the contact elements thereof result in resistances therebetween which are both unstable, and show a worsening trend over time. The abrasion products also generate electrical noise between the components, again with deleterious effect on attempts to maintain a specific resistance between those components.
Contact fingers which contact one end of the rotor shaft of a motor put a pre-load force axially on the shaft. This preload force is transmitted to the bearings of the motor, and results in increased noise and reduced bearing life. Furthermore, increased preload forces induce increased frictional torque. Increasing the preload force in order to maximize electrical connectivity exacerbates these problems.
Finally, slip rings and contact fingers which contact the shaft radially place an axial thrust on the bearings, inducing a radial force which further increases the frictional torque of the motor. This radial force also shortens the bearing's service life. Also implicit in the use of these radial devices is the fact that the further from the center of rotation they are, the larger the swept area they incur, and the more noise, both electrical and acoustical, and abrasion they cause.
In order to overcome the increased frictional torque resultant from either axial or radial loading of the rotor, the motor must generate more rotary torque. To do so increases the size of the motor. This is not a desirable design option in current art disc drive technology.
What is required therefore, to provide a reliable current path or connectivity between the moving and static portions of a precision electrical device is a low-friction contact element, applied at, or close to, the axis of spindle motor rotor rotation. The connectivity would provide for and enable an essentially continuous resistance across the interface between the rotor and stator. This connectivity should not result in any appreciable of electrical or acoustical noise, or in the production of abraded particles.